The invention relates to a nut unit for use in a ball screw drive, comprising a sleeve-shaped nut body that is made of an essentially rigid material and that has an axis and an inner circumferential surface, wherein a one-piece, ball-guiding helix of strip materialxe2x80x94which is channel-shaped as viewed in transverse cross-sectionxe2x80x94is arranged on the inner circumferential surface of the nut body, wherein the helix of strip material is attached to the inner circumferential surface of the sleeve-shaped nut body by a convexly curved external profile, the latter being embodied with external profile flank regions and an external profile crown region, and the helix defines, by means of a concavely curved internal profile facing the axis, a ball channel with an internal profile base region and two internal profile flank regions.
In order to form a ball race, it is known from DE 27 32 896 C2 to apply to the inner circumferential surface of a sleeve-shaped nut body a channel-like, profiled metal strip which is rigidly attached along its entire length to the inner circumferential surface of the nut body. The flanks of the metal strip extend out into the central space of the nut body, which is to say that they are not supported outside of the base region. Moreover, known from DE 27 32 896 C2 is an embodiment (see FIG. 3) wherein a profiled metal strip is attached by vulcanization to the inner circumferential surface of a bushing made of elastically deformable material. The subject there is thus not a nut body of an essentially rigid material, but of an elastically deformable material.
It is known from DE 28 05 141 to cut a helical profiled groove in the inner surface of a sleeve-shaped nut body and to allow the balls of a continuous set of balls to run directly in this profiled groove on the nut side. In this connection, high demands are placed on the profiled groove with respect to the surface characteristics of the bearing race region, in particular with respect to hardness and smoothness.
In order to create a helicoid profiled groove on the inner circumferential surface of a nut body, it is known from DE 30 28 543 to lay a round wire helix on the inner circumferential surface of a nut body and to arrange it in a helicoid manner by means of ribs on the inner circumferential surface of the nut body. Therein, the balls of an associated ball set each run between two adjacent turns of the round wire. It is further known from DE 30 28 543 to form a cylindrical tube into a screw-like helix to form a nut unit and to slit this tube on the inner side of the helix, so that the balls of a ball set carried in the tube can project radially inward and engage the threads of a spindle.
In contrast, it is proposed in accordance with the invention that the outer profile flank regions of the strip material helix be essentially rigidly supported by a support profile of a support groove formed in the inner circumferential surface of the nut body.
The following is achieved by the arrangement in accordance with the invention, in contrast to the state of the art discussed above: in comparison to the first embodiment per DE 27 32 896 C2, an increased stiffness is achieved. Owing to the support by the support groove in the external profile flank regions, deformation of the strip material is in any case largely suppressed if not completely precluded. The strip material can neither tip nor be bent. On the other hand, the surface of the strip material helix in the region of the concavely curved internal profile of the support groove can be finished with respect to hardness and smoothness prior to the installation of the strip material helix in the nut body, and if desired at the strip material itself prior to its forming or at least prior to its final shaping. This has particularly great importance when large thread pitches are required for the ball nut, a requirement that is occasionally present in the case of machine tools in order to be able to increase the feed rates without having to raise the spindle speed to levels that are critical in terms of bending. When large nut thread pitches are required, it becomes increasingly difficult to perform smoothing operations, in particular grinding, on the finished threads as the pitch increases.
Also in contrast to the second embodiment per DE 27 32 896 C2, wherein the metal strip is attached by vulcanization to the inner circumferential surface of a bushing of elastically deformable material, the embodiment in accordance with the invention provides the important advantage, owing to the essentially rigid support of the external profile flank regions by the support profile of the support groove, that the strip material can neither tip nor be bent and cannot yield in any other way, which is precisely the goal of the known embodiment with respect to the change in pitch that is striven for therein.
In contrast to the known embodiment per DE 28 05 141, the effect is achieved that surface treatment with regard to hardness and smoothness in the ball race is possible without regard to the thread pitch, because smoothing and hardening treatments are possible before installation in the nut body and even before shaping of the strip material helix, but in any case prior to its final shaping.
In contrast to the first-described known embodiment per DE 30 28 543 A1, wherein balls are guided on the nut body between two adjacent turns of the round wire helix, the advantage is achieved that balls are guided on the nut body by a one-piece channel profile, resulting in a better and easier to manufacture precision and in higher load capacity. In contrast to the further embodiment described in DE 30 28 543 A1, wherein balls are guided in a tube that is rolled into a spiral and cut on the inside of the spiral, the advantage of increased stiffness is again achieved owing to the support of the external profile flank regions by the support profile of the support groove.
The proposed invention can in particular find application when the concavely curved internal profile is shaped such that balls of an associated ball set of predetermined nominal diameter run on bearing race tracks of the internal profile flank regions, each of which bearing race tracks lie within a track region of the relevant internal profile flank region. In such an embodiment of the ball channel, the strip material helix is stably supported by the support of its external profile flank regions by the support profile of the support groove at or in the immediate vicinity of the bearing race track and/or the bearing race tracks that are possible as a result of altering the ball diameter, so that maximum stiffness of the ball screw drive can be achieved.
When reference is made to a track region, in particular that track region is meant that is determined by the possible bearing race tracks which result from a group of ball sets with nominal diameters graduated from ball set to ball set that are available to set a specific preload range for the ball screw drive. In this connection, the nominal diameter in each case determines the xe2x80x9cpressure anglexe2x80x9d (defined hereinafter).
A first embodiment of the invention resides in that a supportxe2x80x94based on physical contactxe2x80x94of an external profile flank region on the support profile of the support groove extends over a contact zone corresponding approximately to the total extent of the track region along the curved internal profile andxe2x80x94if desiredxe2x80x94extends beyond the borders of the track region. In this first embodiment, all conceivable bearing race tracks that result from changing the nominal ball diameter are supported directly and free of bending on the back side, which is to say in the external profile flank region, so that optimal stiffness is achieved. However, in this embodiment a relatively high precision is demanded in the manufacture of the external profile flank regions and the support profile.
In accordance with a second embodiment of the invention, provision is made for the support, based on physical contact, of an external profile flank region on the support profile of the support groove to be limited to contact zones that correspond to boundary zones of the associated track region and that are spaced from each other in the direction of curvature of the curved external profile and that xe2x80x94if desiredxe2x80x94extend beyond the boundary zones of the track regions. In this second embodiment, the support of each external profile flank region is accomplished in the manner of or similar to a two-point support, each approximately in the boundary zones of the associated track region. One can say that the channel-shaped strip material helix, viewed in cross-section, forms an essentially rigid bridge across an interruption of the physical support. An adequately rigid support can be reckoned with in this embodiment as well when the strip material wall thickness and the spacing of the contact zones are appropriately matched.
In both embodiments a stable support of the channel-shaped strip material helix is provided independent of the particular pressure angle (defined hereinafter).
It is possible that the convexly curved external profile and/or the support profile of the support groove extend essentially without kinks at least over the length of the associated track region. However, it is also possible that the convexly curved external profile and/or the support profile are polygonal or polylobal, at least over the length of the associated track region. In both possible cases, either full-area support over the entire track region or bridge-like support can be chosen.
With regard to the achievement of improved fitting of the relevant balls against the internal profile flank regions, a preferred embodiment resides in that the internal profile curve is essentially ogival in shape at least in its internal profile flank regions. Provision is advantageously made herein that the concavely-curved internal profile and the convexly curved external profile are essentially equidistant at least over the length of the internal profile flank regions of the curved internal profile. The latter measure achieves the result that when manufacturing the strip material helix one can start with a plane parallel or approximately plane parallel flat strip, and the strip material helix receives a channel-shaped cross-section with a minimum of forming work.
With regard to simplification of the forming work during manufacture of the channel-shaped helix of strip material, and also to the placement of the strip material helix on the support profile, it can be advantageous if the strip material in the base region of the concavely curved internal profile is weakened about a base centerline with regard to the bending section modulus. This weakening of the bending section modulus can be achieved, for example, in that the strip material has a recess in the base region of the curved internal profile. The recess can be formed as early as during fabrication of the strip material or can be formed thereupon later, for example with an ogival profile. However, it is also possible to generate the recess during the course of rolling a flat strip into channel profile. Weakening of the bending section modulus facilitates shaping of the channel-shaped helix of strip material into its final form, thus ensuring that the critical surfaces for the ball race and for supporting the channel-shaped helix of strip material can be fabricated with high precision.
Additional measures can be taken to ensure that the strip material in the direction of the convexly curved external profile is secured against shifting in position relative to the support profile, at least along the length of the track region. Thus it is possible, for example, for the strip material to be glued to the support profile. In this event, the adhesive layer can also perform an equalizing function between the strip material and the support profile. Of course, in the event that the adhesive layer is assigned an equalizing function, it is desirable to ensure that the adhesive layer possesses adequate indentation hardness so that deformability of the adhesive layer does not jeopardize the stiffness of the ball screw.
The additional securing of the strip material can also be achieved in that the edges of the strip material that are distant from the base region are fixed to the nut bodyxe2x80x94if desired under preloadingxe2x80x94through positive locking. Such positive locking can be produced by means of recesses in the strip material on its edges distant from the base region. Furthermore, positive locking can be produced by caulking the nut body in the vicinity of the edge regions of the support profile. It is beneficial for the stiffness of the ball screw drive if the strip material is pressed radially outward into the support profile along the entire course of its helix. Some of the pressure can be applied by the balls when they are subjected to preloading between the spindle and the strip material helix, a preloading which in turn is beneficial for the stiffness of the ball screw drive. However, it is also possible for the strip material to be pressed against the support profile of the support groove of the nut body under radial preloading independently of the preloading of the balls, in particular owing to radial overdimensioning of the helical strip material prior to installation in the support groove.
At least in its layer near the inner circumferential surface, the nut body can be made of metal, preferably steel. In this event, the customary thread cutting processes may be used to manufacture the helical support groove.
At least in a layer near the curved internal profile, the strip material helix can be made of metal, preferably steel. Fabrication of the strip material helix out of a flat strip by producing the channel cross-section and by subsequent winding can take place on a conventional spring coiling machine. It is possible for the production of the channel and the winding to take place simultaneously in one step, or to be performed one immediately following the other. Rolling of the recess in the base region can also be included in this step as a preliminary stage.
The support groove on the inner circumferential surface of the nut body can be produced through a thread-cutting process and left essentially without finishing by hardening and grinding. Therein lies a substantial advantage of the invention: if neither hardening nor smoothing of the nut body is necessary after thread cutting, because hardening and/or smoothing is done on the strip material or the partially or fully shaped helix of strip material, the overall fabrication of the nut unit is substantially simplified, in particular for the case of large thread pitch addressed above. This is surprising inasmuch as one could assume in principle that the simplest and most precise manufacture of the ball nut unit would be obtained if one were to simply cut a thread suitable for direct ball guidance in the inner circumferential surface of a ball nut blank.
With regard to minimization of wear and also high carrying power of the spindle screw drive, the helix of strip material should be hardened at least in one layer near the curved internal profile, where it should be hardened at least in the vicinity of the bearing races. This hardening can be done in a simple way prior to installation of the strip material helix in the nut body. In order to have available the most ductile possible strip material when shaping the channeled strip material helix, it is recommended that hardening be done after shaping of the strip material helix is completed or at least partially completed.
With regard to smooth ball travel, and also with regard to high stiffness and machining precision of the ball screw drive, it is desirable for the strip material helix to be smoothed at least in the track region. What is special about the invention in this regard is that the smoothing does not necessarily have to take place after installation of the strip material helix in the support groove, whichxe2x80x94as already mentionedxe2x80x94is difficult, especially when the pitch of the strip material helix is great. Instead, it is possible to undertake the necessary smoothing operations on the intermediate product, for example during fabrication of the strip or when shaping the channel profile, or when winding the strip material into a helix. The smoothing can be achieved more particularly by a rolling treatment, which preferably can take place before any hardening in order to have the advantage during the smoothing process as well of the higher ductility of the material to be worked.
When manufacturing the nut unit in accordance with the invention, it is possible to proceed in that one introduces into an unhardened, sleeve-shaped nut body blank a helical support groove with a support profile using a thread-forming process, in that one forms a strip material into a channel-shaped strip material helix with a smooth surface, at least in the track regions of the internal profile flank regions, and in that one introduces the strip material helix into the helical support groove. The advantage of this process is that a nut unit is manufactured for a stiff ball screw drive with high surface quality of the ball races, even if the machining circumstances are unfavorable, for example because of large pitch for the threads. The strip material helix can be obtained by rolling an initially essentially flat, straight steel strip into an essentially straight channel profile, and subsequently winding the channel profile into a strip material helix.
Hardening of at least the track regions can be done preferably after the formation of the channel profile and the winding process, and preferably before introduction into the helical support groove; conventional processes, for example inductive hardening (penetration hardening) or case-hardening (surface hardening), may be used.
A smoothing treatment can be undertaken more simply before the geometry of the strip material helix has been finalized, for example by means of a rolling treatment before or during formation of the channel-shaped strip material helix.
When manufacturing the strip material helix, it is possible to start with flat strip material or strip material in roll form, which, in the base region, is weakened about a base centerline with regard to the bending section modulus.
The strip material used to form the helix can be produced through drawing or rolling or cutting.
The support profile can be produced using a conventional thread-forming process, more particularly thread cutting process. Subsequent hardening or smoothing of the support profile is not necessary.
The nut unit in accordance with the invention can be equipped with ball sets of different nominal diameters. When this is done, it is desirable to observe the following: the starting point for assembling a ball screw drive or ball screw is the desired preloading of the balls between the spindle and nut unit. The nominal diameter for the balls is a result of the actual dimensions of the spindle bearing race and nut bearing race. If one selects a certain nominal diameter for the balls of the ball set in question based on a certain preloading of the balls, a certain pressure angle ensues. The pressure angle is defined as the angle between a reference plane perpendicular to the axis and a ray from the ball mid-point to the contact point between the ball and race. Of course, the pressure angle also depends upon the manufacturing precision of the spindle and nut. If one wishes to select different preloading stages, one must work with ball sets whose balls accordingly have different nominal diameters. Different pressure angles ensue accordingly.